This invention relates to a magneto-inductive flowmeter for measuring the flow of fluids, incorporating a measuring tube, two magnetic poles externally contacting the measuring tube, and two interconnected return plates, where each magnetic pole is provided with a field coil supporting a sheet-metal pole shoe, each pole shoe connects to a return plate at the open end of the field coil supporting the respective pole shoe and each is provided on the outside of, and extending from, the open end of the respective field coil supporting the associated pole shoe with sections which are in contact with the measuring tube. A flowmeter of this type has been described earlier, for instance in the German patent document DE 35 01 768 C2.
The underlying concept of a magneto-inductive flowmeter for measuring the flow of a fluid, or medium, goes all the way back to Faraday who in 1832 proposed to employ the principle of electrodynamic induction for measuring flow rates. According to Faraday""s law of induction, a moving fluid that contains charge carriers and flows through a magnetic field generates an electrical field intensity perpendicular to the flow direction and perpendicular to the magnetic field. A magneto-inductive flowmeter employs this phenomenon by means of a magnet which, typically consisting of two magnetic poles each associated with a field coil, generates a magnetic field perpendicular to the direction of the flow in the measuring tube. Within this magnetic field, each volume element of the flowing medium that travels through the magnetic field and contains a certain number of charge carriers, contributes, by virtue of the field intensity generated in the volume element concerned, to a measuring voltage that can be collected by way of test electrodes. In prior-art magneto-inductive flowmeters, the test electrodes are so designed as to be either electrically or capacitively coupled to the flowing medium. Of particular significance in magneto-inductive flowmeters is the proportionality between the measuring voltage and the flow rate of the medium as averaged over the cross section of the tube, i.e. between the measuring voltage and the volume flow.
In the prior-art magneto-inductive flowmeter described above, two mutually opposite pole shoes are in essentially full contact with the measuring tube. Each of these pole shoes supports a saddle-shaped field coil into the opening of which protrudes a core section which extends all the way to, and rests against, the pole shoe. These core sections are constituted of sheet-metal segments integrally connected to the return plates which enclose the entire assembly consisting of measuring tube, pole shoes and field coils. The result is a compact magneto-inductive flowmeter capable of producing a constant magnetic field sufficiently strong for magneto-inductive flow-rate measurements, given that there are virtually no air gaps between the pole shoe and the circumference of the tube or in the area of the the core sections which gaps might interfere with the magnetic flux.
However, that prior-art design has a number of drawbacks. For example, producing such conventional magneto-inductive flowmeters is a complex process insofar as it is necessary during the assembly of the individual components of the magneto-inductive flowmeter to mount the field coils on the pole shoes which completely hug the measuring tube, leaving no possibility for the localized attachment of the field coil. Moreover, the core sections which form an integral part of the return plates are mounted on the pole shoe in a recess provided in the return plates, meaning that the opening of the respective field coil which determines the size of the recess must be of a diameter large enough to allow access to the bottom of the recess for a mounting tool, for instance a spot welder or a riveter. This also severely limits the number of windings of the field coil, given that in view of these mounting requirements, the field coil cannot be of as small a diameter as might be desirable.
It is the objective of this invention to introduce a magneto-inductive flowmeter which is easy to manufacture and which can accommodate a large field coil with many windings.
The magneto-inductive flowmeter according to this invention, designed to solve the above-mentioned problem, is characterized in that the pole shoes, located in the open-end area of the field coil supporting the pole shoe concerned, are positioned at a distance from the measuring tube in such fashion that, in the area of the opening of the field coil supporting the individual pole shoes, the pole shoes are provided with sections which protrude into the field coil and extend to the far end of the opening of the pole-shoe-supporting field coil away from the measuring tube. This invention thus makes it possible to attach a radially configured return plate to the pole shoe outside the field coil, providing sufficient accessibility for practically any type of mounting tool. Obviating the need for recess mounting eliminates the minimum size restrictions for the diameter of the field coil so that even field coils with a very small diameter can be employed. By the same token, a field coil with a smaller inner diameter can accommodate a larger number of windings which permit the generation of a larger magnetic field. Moreover, the pole-shoe sections positioned at a distance from the measuring tube and protruding into the field-coil opening allow for a simple and secure placement of the field coil during the assembly of the individual components of the magneto-inductive flowmeter according to this invention.
In a preferred design enhancement of the magneto-inductive flowmeter according to this invention, the sections of each pole shoe located outside the opening of the field coil and contacting the measuring tube connect to a section angled away from the measuring tube. This pole-shoe configuration offers a number of advantages: The sections angled away from the measuring tube add support and stabilization to the field coils which substantially facilitates the positioning of the field coils during the assembly process of the magneto-inductive flowmeter. Also, the sections angled away from the measuring tube define the direction of the magnetic field in the area near the electrodes. Indeed, if in a further, preferred design enhancement of this invention, the sections of the mutually opposite pole shoes, angled away from the measuring tube, are lined up parallel to one another, it is possible to obtain particularly good homogeneity of the magnetic field generated by the field coils over a very wide range and essentially throughout the cross section of the measuring tube. Finally, the sections angled away from the measuring tube electrically shield the electrodes, and thus the measuring voltage collected from the electrodes, from the coil signal. This, in turn, further improves the signal-to-noise ratio of the measuring voltage collected in the magneto-inductive flowmeter according to the invention.
In another, preferred design enhancement of the magneto-inductive flowmeter according to this invention, the pole shoes are provided, at the end of the opening of the pole-shoe supporting field coil away from the measuring tube, with a pole-shoe section which interconnects the pole-shoe sections that protrude into the field-coil opening and extend to the end of the opening, facing away from the measuring tube, of the field coil supporting the respective pole shoe. This allows the pole shoe to be produced as a single unit which substantially improves its stability. It also permits planar contact between the return plate and the pole-shoe section which, in turn, simplifies the connection of the return plates with the pole shoe.
In another preferred design enhancement according to this invention, the field coil is saddle-shaped. This allows the field coil to match the curvature of the pole shoe contacting the measuring tube.
In a preferred design enhancement according to this invention, the magneto-inductive flowmeter can be provided with certain application-specific properties by producing the pole shoes and the return plates from mutually different magnetizable materials.
Finally, in a preferred design enhancement of the magneto-inductive flowmeter according to this invention, the return plates surround the magnetic poles and the measuring tube in peripheral fashion.